1. Field of the Invention
The present invention relates to a signal converting apparatus for switching a plurality of images and generating a composite image, in particular to, a signal converting apparatus for generating a key signal for switching images.
2. Description of the Prior Art
A signal converting apparatus that switches a plurality of images and generates a composite image, a chromakey unit, a switcher, a video effector, and the like are known. In the chromakey unit, a specific color is designated to one image (foreground image) of at least two types of images. The color portion is substituted with another color (background image) so as to generate a compound image.
The color designating switching signal is referred to as a key signal. A means for preventing a switched portion of an image from deteriorating is employed. As an example of factors that cause such deterioration, quantizing noise of a key signal is known. As switching methods of two types of images with a key signal, a hard key method and a soft key method are available. The hard key method is used to switch images with a binary key signal. The soft key method is used to provide intermediate levels. In these methods, a key signal for a designated color contained in a foreground image is compared with a threshold value. Thereafter, images to be switched are enlarged. This process is referred to as a stretching process. The magnification of the stretched image is referred to as stretch gain.
FIG. 10 schematically shows a construction of a conventional digital chromakey apparatus. Two types of image signals, a foreground image signal and a background image signal, are supplied from input terminals 61 and 62, respectively. With the foreground image signal received from the input terminal 61, the region of a specific color is extracted. FIG. 9A shows an example of the extracted signal. In this example, a signal in the range from 0 to threshold value (see FIG. 9A) is converted into a signal in the range from 0 to 255 (see FIG. 9B). In this example, since data is handled with eight bits, it is represented with values in the range from 0 to 255. Each pixel contained in an image signal is composed of eight bits.
In FIG. 10, a threshold value Th is variable. The threshold value Th is supplied from outside of the apparatus to a key signal generating portion 64 through the terminal 63. With the threshold value Th, the key signal generating portion 64 performs a stretch process. A multiplier 65 multiplies the foreground image signal by a coefficient k of the key signal supplied from the key signal generating portion 64. A multiplier 67 multiplies the background image signal received from the input terminal 62 by a coefficient (1-k) of the key signal supplied from the key signal generating portion 64 through a compensation signal generating portion 66. An adder 68 adds the outputs of the multipliers 65 and 67 so as to combine the images. Thus, a composite image where the foreground image is incorporated into the background image is generated. Thus, the composite image is obtained from an output terminal 69.
A composite image generated by the conventional chromakey apparatus is deteriorated because of the stretch process performed for generating a key signal.
In the example shown in FIGS. 9A and 9B, the stretch gain is "5". The quantizing noise is proportional to the stretch gain. In other words, as the stretch gain increases, the image deterioration takes place. When a clear cup filled with water is foreground image and the image is switched with a soft key, a composite image with remarkable quantizing noise that appears inside the glass is generated. As an example of the countermeasures against the quantizing noise, an image with small stretch gain may be selected. As another example, the number of transmission quantizing bits may be increased. However, the method for increasing the number of transmission quantizing bits has a problem on transmission line.
Therefore, an object of the present invention is to provide a signal converting apparatus for performing a stretch process without increasing quantizing noise of an image switching signal.